14 WILLIAM V. ALLEN 



as dorsal and ventral portions of the anterior end of the fourth 

 ventricle. For the reason that in Polistotrema they extend 

 some distance behind the posterior tip of the mesencephalon, 

 and the dorsal or posterior canal is shown in transverse section 

 (fig. 65) to lie close to the dorsal surface, which is then the only 

 part of the fourth ventricle to retain the characteristic dorsal 

 position of the higher vertebrates. These two canals appear in 

 this series about as Nicholls has described them, the dorsal 

 (isthmic) is the larger and contains Reissner's fiber. Although 

 probably subject to a considerable variation, these canals 

 apparently extend further caudad in this series than Nicholls 

 represents them. Also in this series the dorsal canal (isthmic) 

 gives off one or two branches at the level of the posterior tip of 

 the mesencephalon (cerebellum of Miss Worthington) , which 

 run parallel to the main canal, but a little to one side and below 

 (fig. 65, A.iV.). After travelling side by side for some little 

 distance in a mass of spongy ependymal tissue close to the roof 

 plate, they reunite in the dorsal canal, and soon afterward 

 both the dorsal and ventral (isthmic and ventricular) canals 

 unite in a common canal, which is little if any. larger than the 

 central canal of the spinal cord. This constricted portion of the 

 fourth ventricle (fig. 63) continues caudad, rather deep-seated 

 in a mass of loose vascular ependyma, until the posterior end of 

 the medulla is reached, where it expands into a much larger 

 vesicle or sinus, designated as the posterior dilation of the 

 fourth ventricle (figs. 63 and 66, P.aV). Behind this it soon 

 tapers down into the ordinary central canal of the spinal cord. 



A glance at figures 63 to 66 suffices to show that the fourth 

 ventricle of Polistotrema is greatly reduced as compared with 

 that of Petromyzon. This is due probably to the rapid increase 

 of fibers and cells in the lateral plates. Notwithstanding this 

 reduction in size and general alteration in appearance and struc- 

 ture, the walls of the fourth ventricle in Polistotrema, although 

 representing a greatly modified chorioid plexus, are unquestion- 

 ably capable of producing cerebro-spinal fluid. The posterior 

 dilation of the fourth ventricle (fig. 66, P.iV.) contains cerebro- 

 spinal fluid (S.C.F.) in the form of a deeply staining feltwork, 



